Future concepts of electrical discharge machines and other types of machine tools need to be more flexible in satisfying current demands better and quicker, and in simplifying the implementation of any function concerned. The production, testing and maintenance of machine tool systems need to be compatible on an international scale. Necessary for this purpose are reduced material and production costs and as many of the system components as possible need to be suitable for use, for example, in wire-cutting as well as die-sinking EDM machine systems, despite the differences in the requirements. In addition, the same modules need to be suitable for use in high-end and low-cost products. Apart from this, standardized diagnostic routines are desirable to simplify verification of increasingly complex functions.
The increasing demands on still higher productivity and flexibility of, for example, an EDM machine is also forcing the power requirement of pulse generators even higher, whilst, on the other hand, the losses in pulse generation need to be minimized. In keeping with enhanced environmental compatibility the losses of an EDM machine or other machine tool when not in operation also need to be further reduced.
FIG. 2 shows the general configuration of a prior art die-sinking EDM machine. A wire-cutting EDM machine differs from the die-sinking EDM machine actually only in details, but nevertheless most manufacturers make use of totally different concepts for implementation and operation the two types of EDM machines. This applies particularly to the pulse generator involved, where very short but high discharge pulses are needed for wire-cutting, whilst for die-sinking longer discharge pulses of corresponding lower amplitude are used. To date there is still no satisfactory solution for a consistent overall concept.
The configuration of the EDM machine system of FIG. 2 generally involves the following sub-systems or sections: a main power input 1, an electronics cabinet 2, a cable system 3 and a machine 4, i.e. the die-sinking machine as such that carries out the machining of a workpiece. The power cabinet 2 houses an AC voltage module (AC), a DC voltage module (DC), a numerical control (CNC), one or more drive modules (DRIVE), a generator module (GEN) as well as a universal machine control module (CONTROL). Since the full content of the power cabinet 2 is considerably bulky and weighty and the total power loss is of the order of a single-digit kW, the power cabinet 2 is normally sited some distance away from the machine 4. Further, the cabling 3 is usually 2 m to 5 m long. A first cable connects the drive modules (DRIVE) to the axis drive motors of the machine 4 and supplies the motor current, the current for any brakes, as may be provided, as well as diverse sensitive digital signals of the position transducers. These cables are a significant cost factor and if not designed and installed and with due care can easily result in expensive downtime.
A second cable connects the generator module (GEN) to the workpiece to be machined and to an electrode tool of the machine 4. This second cable has the disadvantage that the power losses, particularly in wire-cutting, due to the high effective value of the pulse current, may be as high as 100 W/m. Apart from this undesirable waste of energy this can also result in the machine structure becoming deformed from the heat and thus to workpiece inaccuracies. Currently, the only solution to this problem is a complicated means of water cooling.
Another disadvantage is also involved in the high rigidity of the cables used, typically needing to involve eight coaxial cables in parallel, each of approximately 5 mm2 cross section of copper. Since the cables are connected to moving structure parts of the machine, their rigidity results in flexing of these structure parts in the micrometer range and thus, of course, to corresponding errors in workpiece machining. Still further, the length of the cables determines their capacity. The energy stored in each cable is also discharged at the working gap such that the achievable roughness of the workpiece is limited.
A third cable serves to connect the universal machine control module (CONTROL) to a large number of function units on the machine 4, such as electrovalves, pumps, auxiliary drives, end switches, temperature sensors, safety guards, etc. This third cable likewise results in considerably costs because a great many different conductors are needed, but also because each machine variant needs ultimately a special cable. A further disadvantage may materialize when the machine 4 and the power cabinet 2 are shipped separately to the customer, the many connections of the cable system 3 required on installation constituting an added fault risk.
In the “Proceedings of the 13th ISEM”, Vol. 1, Bilbao 2001, pages 3 to 19, MASUZAWA, all processes and equations fundamental to pulse generation via pulse capacitors are explained as regards their application in micro EDM. These comments apply in general and thus also to the present invention.
U.S. Pat. No. 4,710,603 (OBARA) discloses a generator for an EDM machine operating on the pulse capacitor discharge principle, the basic circuit of which is shown in FIG. 3 of this application. From a DC voltage source E a capacitor C1 is charged via a switching element Q1 and an inductance L3. A further switching element Q2 discharges the pulse capacitor C1 via a further inductance L2 into the spark gap PW. This is circuit requires neither charge resistors nor switching elements in linear operation.
U.S. Pat. No. 4,766,281 (BÜHLER) discloses an EDM machine generator with a passive charge voltage regulator, as shown in FIG. 4 of this application. The charge voltage regulator comprises a flyback converter transformer and two diodes. The efficiency of this generator is high since the commutation losses as occur with the generator as it reads from U.S. Pat. No. 4,710,603 across the switching element Q1 are eliminated.
However, both these generators still have disadvantages. Firstly, the pulse frequency is restricted to modest values of around 70 kHz due to monopolar charging. Increasing the frequency further would allow the charge current to increase to values detrimenting the efficiency. Secondly, the generators are still too large to permit their location e.g. in the direct vicinity of the electrode. For a more detained explanation of this, reference is made to FIG. 5 plotting for these generators the curves of the capacitor voltage Uc and pulse current Igap at the spark gap as a function of time t. It is evident that for a sinusoidal pulse current Igap the negative charge voltage U_chrg flips cosinusoidally to a positive residual charge voltage U_end. This residual charge voltage U_end corresponds precisely to the energy which is not converted in the spark gap and reflected back to the pulse capacitor. Ignoring the line losses the residual charge voltage as it reads from the aforementioned Proceedings of the 13th ISEM Vol. 1, Bilbao 2001, pages 3 to 19 is:U—end=−U—chrg+2*U—gap wherein U_gap corresponds to the voltage across a spark gap formed between a machining electrode and the workpiece. The residual voltage U_end is accordingly a function of neither the pulse current nor of the capacitance of the pulse capacitor, nor of the inductance of the discharge circuit. After a discharge the charge voltage regulator immediately commences to recharge the pulse capacitor again to the desired negative charge voltage U_chrg. In this arrangement, the complete electrical energy of the residual charge voltage U_end is converted within an inductance (e.g. within the coil L3 in FIG. 3 or within the flyback converter transformer in FIG. 4) firstly into magnetic energy, before then being stored again in form of electrical energy in the pulse capacitor in reverse polarity.
EP 698 440 B1 (KANEKO) discloses an EDM power supply system wherein a pulse transformer 13 (in FIG. 1 KANEKO) together with the switching contacts 14A to 14D are housed in a separate case 12 in the vicinity of the spark gap 1, 3. In this arrangement the pulse transformer 13 can be switched active or passive by the switching contacts 14A to 14D. This device is provided for wire-cutting machines for generating bipolar pulses via the pulse transformer 13 and thus to reduce wire vibration in “second cutting”. However, this known generator is still too bulky and subject to high losses, this being the reason why power cables 11, 17 with the discussed disadvantages are still needed for pulse communication from the generator module to the machine.
U.S. Pat. No. 6,080,953 (BANZAI) proposes a modular generator arranged directly surrounding the wire electrode of a wire-cutting machine and cooled by the machining fluid (water) with the intention of reducing the inductance in the working space. Otherwise, the configuration of wire-cutting generators having a proven record of success is adapted and for die-sinking the proposals fail to apply in any case. Still of a disadvantage is the additional power loss in the single-digit kW range which is dissipated via the flushing medium in the work container of the machine, resulting in an undesirable temperature increase in the working space as a whole and thus in problems as to thermal stability which consequently may result in loss of accuracy in machining. Directly immersing the generator modules in the machining fluid as also proposed is unfavorable, because of it accelerating soilage in thus reducing the cooling capacity. Potting the generator modules in a mixture of metal powder and resin likewise proposed to improve heat dissipation also result in problems. Although a metal powder could be processed to produce an electrical insulation, the metal powder will form parasitic capacitances to all components of the generator, resulting in high-frequency dissipation currents into the case and other components. Such a generator would thus be hampered by multiple disturbances. There is also a problem with a potted generator module being impossible to repair, apart from it being extremely difficult to separate these into their individual components for environment compatible disposal.
The aforementioned prior art proposals are thus not suitable in solving the problem for an effective conception of an electrical discharge machining system and other like machine tools.
The present invention is intended to solve this problem in presenting an effective overall concept for a method of operating a machine tool as well as an overall concept for a machine tool system and its manufacturing, in particular for an electrical discharge machining system.